FIG. 1 is a block diagram of a conventional digital amplifier and FIG. 2 is a graph showing a waveform output from a typical Pulse Width Modulation (PWM) modulator.
A digital amplifier, which is a device for amplifying an audio signal in a digital manner, includes a PWM modulator 100 and a PWM amplifier 200 for amplifying a PWM signal. In the digital amplifier, an audio signal is converted to a PWM signal which is a kind of digital signal by the PWM modulator 100 and the PWM signal is amplified by the PWM amplifier 200. Then, the amplified signal passes through a low pass filter 300, so that the amplified signal is converted back to the original analog signal.
In a general class-D amplifier, Pulse Code Modulation (PCM) audio signals sampled in a digital manner are input to the PWM modulator 100. Then, the PWM signal modulated by the PWM modulator 100 is amplified in the PWM amplifier 200, passes through the low pass filter 300, and is then input to a speaker 400 in a form of an analog signal.
The reason why the digital PCM signal is converted to the PWM signal is that the PWM signal has a constant magnitude (or height) and stores information on the original signal into the sample width thereof as shown in FIG. 2, whereas the PCM signal is sampled at a predetermined time interval and each sample contains information on the original signal in the sample size thereof. Because the PWM signal has a constant height (magnitude), it can define two states of 0/1 or ON/OFF and can more easily perform amplification and achieve high output by using an FET switching. The reason why amplifying the PWM signal is that the signal itself from the PWM modulator 100 has too small an output power which is insufficient to directly operate the speaker.
The PWM amplifier 200 generally includes a gate driver and a switching element. As the switching element, Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) suitable for high-speed switching of big electric currents are usually used, which include ‘N channel type’ transistors and ‘P channel type’ transistors.
Further, the PWM amplifier 200 may be classified into a ‘half bridge type’ amplifier and a ‘full bridge type’ amplifier according to the circuit types. In the ‘half bridge type’ amplifier, two Field Effect Transistors (FETS) are vertically stacked and the connection point between the upper FET and the lower FET is connected to the speaker 400 through the low pass filter 300. The ‘full bridge type’ amplifier employs a circuit including a load connected between two half bridge output ports.
Therefore, the PWM amplifier 200 can be classified into an ‘NN half bridge type’ amplifier and an ‘NN full bridge type’ amplifier using only N channel MOSFETs, and a ‘PN half bridge type’ amplifier and a ‘PN full bridge type’ amplifier using both a P channel MOSFET and an N channel MOSFET.
When the amplified PWM signal is suddenly applied to the speaker, it generates pop noise. The pop noise refers to noise generated due to abrupt voltage change when an audio appliance is powered on or off and may cause a user to feel unpleasant. As shown in FIG. 2, because the PWM signal employs the pulse width modulation, abrupt voltage increase is inevitable when the PWM signal is initially applied. Such abrupt voltage increase causes abrupt introduction of electric current to the speaker 400. Therefore, in the class-D amplifier using the PWM modulator 100, it is an important technical subject to reduce the pop noise occurring when the power is applied.
In order to eliminate such pop noise, there has been a method in which a PWM signal is supplied before operation power is supplied to a transistor of the PWM amplifier and the PWM signal is continuously supplied for a predetermined period of time even after the power supply to the transistor of the PWM amplifier is stopped. Korean Patent Laid Open Publication No. 2004-0098925 discloses such a conventional method in detail.
According to another conventional method for eliminating pop noise, a switch such as a relay may be used. In this method, the relay is a mechanical switch which allows the PWM signal to be transferred to the speaker only after passage of some time from the time point when the PWM signal is applied. However, the relay inevitably increases the price of the product equipped with the relay. Therefore, it is impossible for a low-priced audio amplifier to employ the relay.